1. Field of the Invention
The present invention is generally concerned with wheel hubs having a built-in brake assembly, of the kind used, for example, in fitting out industrial vehicles, agricultural machinery, civil engineering equipment and materials handling equipment such as cranes and travelling overhead cranes.
2. Description of the Prior Art
Generally speaking, a wheel hub of this kind comprises a generally tubular hollow body rotatably mounted on bearings, usually two in number, on a fixed support sleeve, commonly referred to as a stub axle, being closed by a removable cover on the side opposite the stub axle and having an external transverse shoulder, in practice a flange, to which a wheel may be attached by its rim flange, there being within the hollow body, in addition to a gear assembly which is adapted to reduce the service rotation speed of the hollow body to an acceptable value and which to this end comprises planet wheels operative between a sun shaft constrained to rotate with a wheel shaft rotatably mounted in the stub axle and an annulus constrained to rotate with a transverse flange, commonly referred to as the annulus carrier, itself constrained to rotate with the stub axle, at least one brake disk which is adapted to be constrained to rotate with the wheel shaft and to be clamped in the axial direction under the control of a hydraulic control unit.
In practice, a number of brake disks are usually employed, alternating with fixed intermediate disks.
The present invention is more particularly, but not necessarily exclusively, directed towards the case where, for the purposes of two-fold control of the so-called built-in brake assembly thus constituted, the hydraulic control unit of the latter comprises two annular pistons each of which, independently of the other, is mounted so as to be axially movable within a respective annular cylinder and one of which, hereinafter called the service piston, is adapted to provide for the utilization of the so-called built-in brake assembly under normal service conditions, in order to brake the vehicle or machine concerned during its movement, whereas the other, hereinater called the standby or parking piston, is adapted to provide for occasional utilization of the brake assembly, in the case, for example, of a failure of any kind affecting the vehicle or machine and/or as a safety measure when the vehicle or machine is parked.
There are normally a number of constraints to be met in producing a wheel hub of this type.
First of all, the overall axial dimension of the assembly must be as small as possible.
There are two reasons for this.
One is to avoid the application of excessively high stresses to the bearings disposed between the hollow body of a wheel hub of this kind and the corresponding stub axle. To protect these bearings, it is desirable for them to be as close as possible to the transverse plane of the wheel concerned, or "track", passing through the median area of the tread of the latter.
The other reason is that, because of economic considerations, it is desirable for a wheel hub of this kind, because of its thus reduced axial dimension, to be equally suitable for use at the end of a rigid axle or at the end of a steering axle, in the latter case meeting the constraint in respect of the clearance required between the wheel disk of the wheel concerned and the ball-joint of the suspension of the wheel.
To meet this requirement for reduced axial overall dimensions, it has already been proposed to move towards one another in the axial direction the two bearings disposed between the hollow body of the hub and the stub axle, or even to combine them, and to dispose at least part of the hydraulic control unit of the built-in brake assembly radially in line with the bearings.
This is the case, for example, in French patent application No. 81 06903 filed on Apr. 3, 1981 and subsequently published under the No. 2 479 746.
However, apart from the fact that in this French patent application the brake has only one control mode, its hydraulic control unit comprising only one piston, in practice a service piston, because the space available radially in line with the bearings is reduced by virtue of the fact that they are contiguous, the hydraulic control unit extends partly within the stub axle and partly within the annulus carrier of the associated gear assembly, complicating the assembly and compromising its reliability.
In the international patent application No. PCT/US 80/00644 filed May 28, 1980 and subsequently published under the number WO81/03469, the hydraulic control unit is also disposed radially in line with one of the two bearings disposed between the hollow body of the hub and the stub axle, to this end being entirely contained within a housing formed to this end within the stub axle.
However, in this international patent application No. PCT//US 80/00644, in addition to other disadvantages to be commented on hereinater, the built-in brake assembly is itself disposed within the stub axle, so that the stub axle must be axially extended for this purpose, the two bearings disposed between it and the hollow body of the hub must necessarily be widely separated from one another, the brake disks of a brake assembly of this kind, being disposed within the stub axle, must necessarily be of relatively reduced diameter, so that to obtain a given overall frictional torque from the brake assembly the number of disks must be increased, compromising the axial dimensions, and the service piston of the hydraulic control unit of the brake assembly is immediately in contact with the brake disks, being thus exposed to the heat generated by these during braking, with the risk of this heat being transmitted to the hydraulic fluid with deleterious consequences.
Another requirement to be satisfied to the greatest possible extent in producing a wheel hub of the kind concerned is to provide easy access to the interior of the hollow body of the latter, to facilitate replacement of the brake disks, for example, and any work which may be necessary on the corresponding hydraulic control unit.
In the French patent application No. 81 06903 mentioned hereinabove, the cover closing the hollow body of the hub at the end thereof opposite the stub axle is to this end advantageously independent of the shoulder on the outside of the hollow body for attaching the wheel disk of the wheel concerned, this cover having a diameter which is less than that of the shoulder and being removable without it being necessary first or conjointly to remove the wheel.
This does not apply in the international patent application No. PCT/US 80/00644 also mentioned hereinabove, in which, to the contrary, since the same screws attach the hollow body of the hub, the cover closing same and the wheel disk of the wheel concerned, removal of the cover requires removal of the wheel.
Also, in this international patent application, the housing in which the hydraulic control unit of the built-in brake assembly is disposed is open in the axial direction on the side of the stub axle opposite the cover closing the hollow body of the hub so that, to gain access to the hydraulic control unit, it is necessary to operate from the opposite side of the stub axle, that is to say from the side towards the chassis of the vehicle concerned, which is particularly difficult.
A general object of the present invention is an arrangement which circumvents the aforementioned disadvantages, provides an advantageous means of satisfying the requirements briefly outlined hereinabove and provides further advantages.